std::trie: Fix find_mut for non-present keys

Make TrieMap/TrieSet's find_mut check the key for external nodes.
Without this find_mut sometimes returns a reference to another key when
querying for a non-present key.
auto merge of #10639 : jix/rust/fix_find_mut_in_trie, r=thestinger

Make TrieMap/TrieSet's find_mut check the key for external nodes.
Without this find_mut sometimes returns a reference to another key when
querying for a non-present key.